1. Field of the Invention
This invention relates to machine tool calibrations, and more particularly, to the calibration of the angular positioning ability of a machine tool table when used in combination with a conventional laser interferometer metrology system.
2. Description of the Prior Art
Machine tools such as but not limited to CNC (Computerized Numerical Controller) milling machines and lathes are well known devices for shaping solid pieces into manufactured parts. As the complexity and tolerances of such parts increase, the calibrations of the positional accuracy for such machine tools must likewise increase. The present invention is useful for making rotary calibrations of CNC machine tools of the type having a table which rotates to prescribed angular positions about a rotary axis normal to the table.
It is well known to make rotary axis calibrations of the CNC machine tool table via a combination of an angular position-measuring laser interferometer system and an indexing rotary calibrator having a so-called Hirth coupling. The indexing rotary calibrator (also known as an indexing rotary table or a circle divider) typically includes upper and lower disks (platens) mounted on a common shaft so that one or both of the disks can rotate about the shaft. The disks are also mounted to move longitudinally back and forth on the shaft relative to one another. On each opposing face of the disks, there is a circular periphery of serrated teeth which precisely mate against one another to form the so-called Hirth coupling when the disks are locked (engaged) together. Each tooth is formed with great precision and the total number is chosen so that the angular positioning of the disks with respect to one another can be within a prescribed high level of tolerance. A standard model rotary calibrator is available from the A. G. Davis Gage and Engineering Company of Hazel Park, Mich., having a coupling indexing accuracy of .+-.0.2 arc second. Other available models have indexing accuracies that range from 0.4 to 1.0 arc second. Another vendor of indexing rotary calibrators is the AA Gage Division of Baldwin, Inc., located in Ferndale, Mich.
Angular position-measuring laser interferometer systems are also well known for making accurate measurements of changes in rotary positions. The HP 5528A Laser Measurement System and the HP 5529A Dynamic Calibrator, which are products made and sold by the Hewlett-Packard Company, the assignee of the present invention, are devices capable of making position measurements well within the modern tolerances desired for machine tools. However, since the optics of the interferometer systems have a limited angular range for a fixed setup, such systems are not suited for making full 360 degree rotary calibrations. In addition, measuring errors accumulate and become larger as the angular distance traveled by the measuring optics increases so that such laser interferometer products are most suited for measuring relatively small angular excursions and are not suited for relatively large angular distances such as would be needed for a full 360 degree calibration of rotation.
The combination of an indexing rotary calibrator with a laser interferometer measuring system is an assemblage known in the prior art for making rotary calibrations of machine tool tables. Typically, rotary calibrations using such commercially available assemblages are usually conducted in the following manner. The lower disk of the calibrator is fixed to the rotary machine tool table being calibrated and the upper disk is held fixed in angular position either by an operator making the calibration or by an external arm mounted in a location not affected by rotary movement of the machine tool table. The axis of rotation of the calibrator disks coincides with the axis of rotation of the machine tool table. The laser interferometer system is set up so that a portion of the optics for measuring angular excursions is coupled to the upper disk. After the indexing rotary calibrator and the laser interferometer measuring system are both put into a predetermined zero initial position, the calibrator is unlocked (that is, the Hirth coupling is disengaged). The machine tool table is then moved to a prescribed rotary position to be calibrated while the upper disk is held in its fixed angular position. Next, the two disks are subsequently locked together. Since the lower disk is fixedly mounted to the machine tool table, any rotation of the upper disk off of its initial zero position as it re-engages the lower disk is the amount of angular error between the prescribed rotary position of the machine tool table and the actual position. Because a portion of the laser interferometer optics is fixed to the upper disk, any corresponding angular motion of the optics is measured by the laser measuring system and a calibration reading can thus be made. Of course, the error measurement is subject to the tolerances existing in the Hirth coupling and the laster interferometer system. The above-described procedure is then repeated for each of the respective angular positions to be calibrated.
A typical calibration standard requires calibration readings to be obtained at 10 degree segments over the 360 degree range of rotary motion and for each of the resulting 36 readings to be repeated at least 6 times. As can be understood and is well known, when the above-mentioned calibration procedure is conducted manually, much time and tedium is involved. With the rising costs of labor, such time also means more expense associated with making such important rotary calibrations.
In an effort to reduce the time, there are prior art commercially available mechanized assemblages which have pneumatic, hydraulic, or electrically motorized mechanisms for locking and unlocking the upper and lower disks of the indexing rotary calibrator. Other commercial assemblages include disks that are servo controlled. By using a programmable computer and associated interface equipment for controlling the indexing rotary calibrator, for inputting commands to the machine tool table, as well as for collecting data from the laser interferometer measuring system, the process for making rotary calibrations is known to contain a high degree of automation. However, such mechanized assemblages are substantially more complicated and thus more expensive than the manually operated rotary indexing calibrator. In order to protect the relatively expensive Hirth coupling from damage as the serrated teeth are locked or unlocked, elaborate safety measures and devices must be included to prevent inadvertent damage. Moreover, with the higher complexity of such mechanized assemblages come attendant problems of reliability and repeatability. Oftentimes because of limited budgets or expense reduction programs, there is a tradeoff between cost and calibration accuracy.